where \(q (x,\mu ^2)\)  is the quark PDF, \(P_{\gamma \leftarrow q}\)  is the \(q\rightarrow \gamma \)  splitting function, and \(P_{\gamma \leftarrow \gamma }\)  corresponds to the virtual self-energy correction to the photon propagator.
This is the basis for colinear photon-PDFs in the initial [1]}, [2]} and more recent [3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]} analyses.
